1. Field of the Invention
The present invention relates to the art of detecting nitrogen oxides, more particularly to a method of detecting nitrogen oxides and also a nitrogen oxide detecting sensor element usable in such method.
2. Description of the Related Art
Background Art Relating to Detection of Nitrogen Oxides
Nitrogen oxides are substances which are generated by oxidation of nitrogen present in the air or in a fuel in the course of combustion of the fuel and which are a cause of air pollution. Hence, there has been a strong demand for prevention of their generation and elimination. For restricting emission of nitrogen oxides, needle to say, it becomes necessary to detect/determine their concentrations. For instance, in employing a preventive method by means of adjustment of combustion conditions, the concentrations of nitrogen oxides present in the flue gas need to be monitored in a continuous manner, and the combustion conditions need to be adjusted based on its result. Thus, the art of detecting nitrogen oxide concentrations with high sensitivity is of great importance.
As such nitrogen oxides detecting art noted above, there have heretofore been known electromotive force type sensors utilizing the principle of a concentration cell and semiconductor type sensors using tin oxides (SnO.sub.2). However, the known sensors of the electromotive force type, though these provide good sensitivity for nitrogen dioxide (NO.sub.2) among the nitrogen oxides, suffer difficulty in detecting NO at lower concentrations. Although the detection of NO too will become possible if NO is converted to NO.sub.2, this will result in complexity of the measuring system.
On the other hand, the semiconductor type sensors using tin oxides can determine gas concentration through measurement of its electron conductivity, so that these sensors tend to have a relatively simple construction as a whole. However, as these sensors have the problem of selectivity, as the tin oxides used therein exhibit sensitivities also for other gases than NO, contained in the exhaust gas, such as the sensitivity for carbon monoxide (CO).
Under the state of the art described above, the present inventors have proposed detection of nitrogen oxides by utilizing semiconductor properties of copper complex oxides such as Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+z, which oxides are known as superconducting materials.
Under such circumstances as above, the present inventors studied the possibility of nitrogen oxides detection with various oxides containing Bi. The result, as described hereinafter, showed that for the nitrogen oxides detection, those oxides containing more than a predetermined ratio of Bi specifically sense nitrogen oxides and show resultant change in their electron conductivity if these materials are placed not under a temperature range where they exhibit oxygen-ion conductivity but under a lower temperature range where they exhibit electron-conductivity. The present invention was completed based on this finding. In this respect, it is noteworthy that the material according to this invention has specifically sensitive to nitrogen oxides, while being hardly sensitive to CO, H.sub.2 which tend to act as interfering gases in the gas detection.
The present inventors achieved the present invention in the manner described above,. Next, further description will be made on the background art relating to gas detecting capabilities of bismuth oxides.
Background Art Relating to Gas Detecting Art Using Bismuth Oxides
Bi.sub.2 O.sub.3 has long been known to have oxygen-ion conductivity. In order to increase its oxygen-ion conductivity, extensive studies have been made on Bi.sub.2 O.sub.3 added with foreign atoms (1 T. Takahashi, H. Iwahara and Y. Yanagi, J. Applied Electrochemistry 2 (1972) 97-104, 2 H. Iwahara, T. Esaka and T. Sato, J. Solid State Chemistry 39, 173-180 (1981), 3 T. Takahashi, H. Iwahara and T. Esaka, J. Electrochemical society (1977) 1563-1569) and on complex oxides containing Bi (4 O. Joubert, A Jouanneaux et. al. Solid State Ionics 73 (1994) 309-318, 5 F. Krok, W. Bougusz et. al Solid State Ionics 70/71 (1994) 211-214). Bi.sub.2 O.sub.3 is formed as a .delta. phase (cubic) at temperature higher than 730.degree. C., which is the phase transition point, so that it exhibits high oxygen-ion conductivity, but it is formed as an alpha phase (monoclinic) at temperature lower than 730.degree. C., so that its oxygen-ion conductivity is lowered and its p-type conductivity becomes dominant. The biggest subject in the study on this material has been to increase the oxygen-ion conductivity by stabilizing the .delta. phase by means of additives (Ba, Ca, W, Nb, Ln, etc.).
Accordingly, when this material is used for gas detection, it has been employed solely in the solid electrolyte type gas sensors. Concerning sensors based on the other detection principles, reports have been limited to those concerning a semiconductor type sensor using complex oxides containing other metallic elements than Bi More specific explanation will be given next.
1) those utilizing oxygen-ion conductivity
As an example of application of oxygen-ion conducting material to gas sensors, there are known an oxygen-ion sensor for deriving an electromotive force from a partial pressure difference of oxygen between two electrodes (Japanese patent application Kokai No. Sho. 58-15067) and a sensor having electrodes thereof coated with catalytic layers for allowing selective transmission of a detection-target gas, thus providing sensitivity for the gas. As the oxygen-ion electrolytes employed in these sensors, zirconia is best known. Similar reports have been made regarding bismuth compounds as well In this case, it is necessary to differentiate the oxygen potentials adjacent the surfaces of the two electrodes by providing a partition wall between the opposed electrodes formed on the solid electrolyte so as to separate the detection-target gas and a reference gas from each other or by coating the electrodes with catalysts having differing catalytic activities, so that the element construction tends to be complicated and the costs tend to be high. Further, as the output varies significantly with variation in the oxygen partial pressure, a system will be needed for high-precision control for maintaining the oxygen partial pressure stationary.
With the sensor of this construction, the target gas for detection comprises oxygen or like inflammable gas (CO, H.sub.2 or the like). And, as such material exhibits its oxygen-ion conductivity at a temperature higher at least than 400.degree. C., the gas detection is effected at some temperature range higher than said temperature range.
2) those utilizing electron-conductivity
The sensor of this type of construction is referred to as the semiconductor type sensor. In comparison with the solid electrolyte type, this has a simpler element construction which allows its system designing at lower costs. However, there has not been known any art of using Bi.sub.2 O.sub.3 per se at its electron-conducting range for detecting a gas (especially nitrogen oxides). The reason for this seems to be that Bi.sub.2 O.sub.3 has conventionally been regarded as near-insulating material rather than as semiconductor material. Namely, since B.sub.2 O.sub.3 has a very high resistance value at the temperature range (the temperature range relating to the present invention from the room temperature to 400.degree. C.) where it behaves like the p-type semiconductor, studies about its gas sensitivity properties have been nonexistent at all.
On the other hand, corresponding to the above-described material that the present inventors have researched, reports are available concerning use, as a nitrogen oxide sensor, of copper type complex oxides such as Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+z which contains Bi and Cu as parts of its crystal structure and which is known as a superconducting material (Japanese patent application Kokai No. Hei. 8-21814, Japanese patent application Kokai No. Hei. 8-271467). These materials have the p-type conductivity and provide a higher resistance value (decreased conductivity) when entrapping a nitrogen oxide therein than otherwise. These nitrogen oxide sensor materials having such p-type conductivity comprise complex oxides or compositions which invariably contain copper as an essential component thereof. Hence, the unique interaction thereof has been attributed to the presence of copper therein. For these reasons, as for the material which does not contain Cu in its crystal lattice and contains a relatively high ratio (50 at % or more in terms of metallic element-based conversion) of Bi having no superconducting property, no studies have been made on its sensitivity properties for nitrogen oxides.